1. Field of the Invention
The present invention relates to ink jet printing apparatus of the continuous type and more specifically to simplified ink circulation systems that supply ink to, and return ink from, the printing station of such apparatus.
2. Description of the Prior Art
In "continuous" ink jet printing apparatus streams of uniformly spaced ink drops are created by imposing periodic perturbations on liquid ink filaments issuing from an orifice plate. The filaments are formed by supplying ink under pressure to a print head cavity that is in communication with the orifice plate. Information is imparted to the droplet streams by selective non-charging or charging and deflection of droplets. A portion of the droplets pass to the recording medium but there are a substantial number of non-printing droplets which are intercepted by a catcher for recirculation. It is often desirable that the print head cavity have an outlet other than the orifice plate (e.g. to facilitate dynamic pressure control within the cavity at start-up), and, in these embodiments, efficient circulation of ink flow from such print head outlet should be accommodated.
Ink drop uniformity requires maintaining a uniform pressure in the print head cavity and this is one primary concern in an ink circulation system. However, it is also very important that unused ink be circulated reliably. For example, if ink intercepted by the catcher is not reliably withdrawn and circulated, an accumulation of ink in the catcher region can impede the path of printing drops and/or cause ink-drip from the catcher onto the print medium.
In view of the above and other considerations, prior art approaches for ink circulation in continuous ink jet printing apparatus have been fairly complex. A typical approach is to utilize a supply pump, under the control of a pressure or flow rate feedback system, to assure proper dynamic print head pressure and a separate pump for maintaining a vacuum in the ink supply reservoir to return ink (e.g. from the catcher or print head outlet).
There are disadvantages connected with the use of such a vacuum pump return system. First, the vacuum pump's continual withdrawal of air from the supply reservoir can cause undesired changes in ink viscosity. Also, the use of separate supply and withdrawal pumps adds cost and complexity to the ink jet printing apparatus, and to its size and energy usage.